Behaviour for delivering import lifecycle events to a host application.

A notifier is called for side effects only: logging, metrics, a webhook, a PubSub broadcast. It is never consulted for a decision and its return value is ignored. It must not be able to fail an import: GeoGenius.Pipeline wraps every notify/3 call so that a notifier which raises or exits has its event logged at warning level and dropped, and the phase continues regardless. This behaviour does not itself guarantee that -- a notifier that raises still raises here -- the guarantee lives entirely in how the pipeline calls it.

events/0 returns the fixed list of events a host can receive, so a host notifier can pattern-match exhaustively instead of guarding against an open-ended atom:

GeoGenius.Pipeline emits the first four on every import, and :release_published when it is asked to publish what it just verified with publish: true. GeoGenius.publish/2 emits :release_published for an explicit publication and GeoGenius.rollback/2 emits :release_rolled_back, each after the write it reports has committed.

A payload key may arrive nil when the write committed but the read that fills the key did not: :collection_key on :release_published, and :release_id on :release_rolled_back. The event fires regardless, because the write it reports happened.